There has been a long-felt need for a non-contact system for accurately measuring a fractional distance between two substantially parallel line or slit elements; the total magnitude and direction of displacement of a reticled measuring device, such as an encoder, including fractional resolution of a final position between two adjacent lines of a reticle; the distance from a reference element to a desired position on an article as, for example, for an operation, such as a spot weld, in a production process; and the like, wherein the system is relatively simple and provides a greater degree of fine resolution and smaller margin of error than is generally available in the present state of the art.
Various means have been employed to provide increasingly fine distance resolution. These include, for example, maximizing reticle density on a linear encoder or on the outer periphery of a disc encoder. This has limitations imposed by such factors as the maximum practical diameter of the disc, maximum practical line density possible in the manufacturing process, and exponentially-increasing cost with increasing disc size and reticle density. Optical and electronic systems have been devised for resolution of fractional distances between two adjacent reticle elements. These generally require the use of at least two separate optical systems with associated electronics, such as at least two slit detector arrays, which require even higher element densities than the article, which must be delicately positioned spatially (with associated error) and which, generally, can at best resolve only equally-sized increments, thereby limiting fineness of resolution. With such prior art fraction resolution systems, it is very difficult to determine accurately multiple element displacement.
The present invention, by use of a single laser interferometer system, can measure lateral displacement from a single element, any fractional distance between two elements, and total displacement from one position to another, including multiple adjacent element integers plus any fractional distance from the last determined element, in a single operation, which also includes a determination of the direction of lateral displacement or distance and the accurate determination of smaller distances than has hitherto been possible.
Laser Doppler Velocimeters (LDV) have recently been developed for determining the rate of fluid flow in wind and water tunnels by suspending small particles in the fluid and determining their velocity and size by means of the velocimeter. Such velocimeters generally comprise convergent laser beams of equal size, intensity, and frequency which produce a stationary interference fringe pattern within the zone of convergence, sometimes called the probe volume. The interference fringes are planes which are normal to the plane defined by the center lines of the two converging laser beams and parallel to the bisector of the converging beams. In operation, the apparatus is set up so that the fluid-borne particles move across the fringes in trajectories normal to the fringe planes, the radiation scattered by the moving particles is optically collected, separated electronically into AC and DC signal components, and the AC/DC ratio is used as a means of determining the size of the particles. Such Laser Doppler Velocimeters are described in detail in the article by W. M. Farmer, "Measurement of Particle Size, Number Density, and Velocity Using a Laser Interferometer," Applied Optics, Vol. 11, No. 11, Nov. 1972, pp. 2603-2612, and G. J. Rudd, U.S. Pat. No. 3,680,961.
In more recent development of the Laser Doppler Velocimeter, the art discloses the use of probe volumes in which the fringes are caused to move continuously in a direction normal to the fringe planes by employing converging laser beams of the same intensity but slightly different frequency, the frequency difference .DELTA.f being within the radio frequency band. Such shifting of the frequency of one of the beams can, for example, be produced by diffraction of an incident laser beam by means of an ultrasonic Bragg cell, which can be made to divide the incident beam into two diverging beam components of the same intensity, one nondiffracted component having the incident beam frequency and the other diffracted component with its frequency shifted by an amount equal to the Bragg cell frequency. Since the two coherent light beams which leave the Bragg cell are diverging, it is required that the beams be converged by an appropriate optical system to form the desired interference fringe pattern. The moving fringe pattern moves at a rate equal to .DELTA.f which in turn is equal to the Bragg cell frequency. As in the case of stationary fringes, the AC/DC ratio technique is employed to determine particle sizes. The application of single and two-dimensional Bragg cell systems to the LDV is disclosed in Chu et al., "Bragg Diffraction of Light by Two Orthogonal Ultrasonic Waves in Water," Appl. Phys. Lett., Vol. 22, No. 11, June 1, 1973, pp. 557-59; and W. M. Farmer et al., "Two-Component, Self-Aligning Laser Vector Velocimeter", Applied Optics, Vol. 12, No. 11, Nov. 1973, pp. 2636-2640.
None of the available art recognizes or discloses the present invention, its principle of operation, or its use in sensing the magnitude and direction of lateral displacement of or lateral distance from a line or slit element of an article. The present invention utilizes known fringe spacings (which can be calculated or otherwise determined by conventional art techniques), a fringe pattern moving at a rate determined by the .DELTA.f radio frequency, relative lateral shifting of the zone and element, and continuous comparison of the RF (AC) component of scattered or transmitted radiation produced by the at least one element with a .DELTA.f RF reference to determine the magnitude and direction of phase shift between the RF radiation signal and the RF reference from one position of the element to another within the fringe zone.
Because the measurement does not require absolute measurement of laser light intensity, but only the detection of the RF component of scattered or transmitted radiation and its phase shift comparison with a constant RF reference, the process and apparatus of the invention have additional advantages including but not limited to the following. Accuracy of measurement is largely independent of intensity fluctuations of the laser source. Accuracy is not affected or compromised by the reflectivity or refractivity of the line elements. Accuracy does not depend on the calibration accuracy of the signal detector devices or the distortions or nonlinearities of components of the optical system, either per se or in terms of sensitivity to changing environmental conditions. Thus, the system and components can be relatively low-cost and can be used in uncontrolled environments, such as manufacturing facilities. In the case of structures with periodically spaced elements, such as linear or angular encoder plates, slight errors in periodicity of the elements are averaged out because a relatively large number of adjacent elements can be simultaneously sensed in the fringe zone at any given time.
It will be understood that the terms "line" and "slit," as used in this specification and claims, include both continuous and discontinuous straight lines or slits. In the latter case, it is essential only that the centers of discontinuous components making up the line or slit be arranged along the same longitudinal axis and the fringe period be substantially wider than the widest component, or the positive difference between the width of the component and a whole integer multiple of the fringe period. Within these limitations, the components can be of any desired shape and size, e.g., rectangular, triangular, trapezoidal, round, oval, and the like.